Surface foaming and sediment deposition can both create problems in storage and treatment tanks where large volumes of liquid/slurry are stored and stirred, mixed, or agitated. In many cases the two problems are inversely related. That is, solving one of the problems may directly worsen the other. This relationship can make it very difficult for those skilled in the art to design and implement a system which addresses and solves both problems.
For example, in activated sludge secondary treatment plants, slurries in large tanks are typically stirred, mixed, or agitated in order to suspend solids in the liquid prior to emptying the tank. Without such mixing, the solids would settle to the bottom of the tank. The settling of solids over even a short period of time can develop into a huge problem, as the build-up of sediment on the tank bottom reduces the volume of the tank. Other benefits of the auxiliary mixing of digester tank contents are: reduction of thermal stratification; dispersing substrate for better contact with active biomass; reduction in scum buildup; dilution of inhibitory substances or adverse pH and temperature feed characteristics; increased effective volume of the reactor; and, separation of reaction product gases is improved. However, stirring, agitation and mixing can also result in increased surface foam due to the abundance of surfactants which can be found in such liquids.
Similarly, surface foaming can be a problem in anaerobic digesters, where gases are the natural product of the digestion process. The gases create foam in the form of bubbles and/or scum on the surface of the liquid/slurry in the tank. If the foaming problem is not addressed, the foam uses up volume in the tank, or the tank can overflow. Further, agitation of the tank contents to assist the digestion process may exacerbate the foaming as noted above.
In both cases, anti-foaming agents may be added to the tanks to suppress foaming. However, these agents are expensive and, in some instances, are either too limited in their ability to suppress foaming or may have negating effects on other favored processes (e.g., digestion).
Another common problem for many activated sludge plant digesters is the creation of a thick (viscous) upper surface. This happens because thickened sludges from the aerobic digestion side of the plant get fed back into the anaerobic digesters to further break down the sludge. This makes the anaerobic digester sludge much more viscous and harder to mix, sometimes resulting in lower level tank mixing but not in upper surface mixing. Floor-mounted mixing nozzles do not necessarily assure upper surface rotation.
Mechanical anti-foaming devices, such as the nozzle system disclosed in U.S. Pat. No. 7,628,183 to Dorsch et al. and assigned to the Assignee of the present technology, are very effective at suppressing foaming. The complete disclosure of U.S. Pat. No. 7,628,183 is hereby incorporated by reference. However, without surface rotation, and due to the extensive size of digester tanks, expensive plumbing for a plurality of anti-foaming nozzles would be required to adequately suppress foaming.
The present invention overcomes these and many other disadvantages of previous devices and processes. Disclosed is a system which is effective at suppressing foaming, even during mixing, stirring and agitation, without the use of expensive anti-foaming agents and the system is easy and relatively inexpensive to manufacture and install.